Membrane Distillation Apparatus and Methods

Abstract

Membranes for membrane distillation include at least one hollow fiber porous hydrophobic membrane, the at least one membrane including carbon nanotubes incorporated into the pore structure of the membrane. Membrane distillation systems may include a heat exchanger operably connected to a hollow fiber membrane module with one or more membranes including carbon nanotubes. Methods of solvent removal, sample preconcentration and desalination employing hollow fiber porous hydrophobic membranes with carbon nanotubes are disclosed.

Description

CROSS-REFERENCE TO RELATED APPLICATION(S)[0001]This application claims the benefit of U.S. Provisional Patent Application No. 61 / 323,860 filed Apr. 13, 2010, the entirety of which is incorporated herein by reference.FIELD OF THE INVENTION[0002]The present invention relates to membrane distillation and in particular to carbon nanotube enhanced membrane distillation apparatus and methods.BACKGROUND[0003]There has been significant interest in using membranes separation for both industrial scale processes and the preparation of analytical samples. In part, this is due to the fact that membranes facilitate extraction and separation without allowing the mixing of two phases. Membrane distillation (MD) is a low temperature (60-90° C.) distillation process that has been used in diverse applications ranging from water desalination to concentration of fruit juice and sucrose. Conventional distillation relies strictly on thermal vaporization, whereas in MD the water vapors are selectively tran...

AI Technical Summary

Benefits of technology

[0006]The present inventors have found that incorporating CNTs into the structure of a hollow fiber polymeric membrane provides unexpectedly enhanced membrane distillation (MD). This is applicable to a wide range of membranes that may be flat, porous, nonporous, composite, hydrophilic or hydrophobic. Other forms of nano carbons can also be substituted for carbon nanotubes. The membranes can be used in the different forms of membrane distillation including Vacuum Gap MD, Direct Contact MD, Airgap MD and Sweep Gas MD.

[0007]In accordance with a first embodiment an improved membrane for membrane distillation is provided wherein nano carbons such as carbon nanotubes (CNTs) are incorporated into the structure of a membrane such as a hollow fiber porous hydrophobic membrane. Recent work has shown CNTs to have excellent sorbent properties as well as desirable mechanical and electrical properties. Popov, V. Mater. Sci. Eng., R, 2004, 43, 61; Baena, J. R et al., Anal. Chem, 2002, 125,1495. CNTs also increase functional surface area in the membrane system due to their high aspect ratio Li, J.; Ma, P. C.; Chow, W. S.; To, C. K.; Tang, B. Z.; Kim, J. K. Advanced Functional Materials, 2007, 17, 16. Since CNTs are non-porous they also exhibit quick desorption. The present inventors have found that incorporating nano carbons such as CNTs into membranes provides alternate transport mechanisms for enhanced separation. The CNTs can act as both molecular transporters and sorbents thereby increasing the permeability of a substance through a membrane as well as increasing the selectivity. Further, CNTs can increase the rate of mass transfer at the liquid membrane pore interface by allowing rapid solute exchange on the CNTs themselves.

[0010]In accordance with one objective the present disclosure provides a membrane distillation method for waste treatment as well as sample preparation for organic compounds including those of pharmaceutical origin. One aspect of the present disclosure provides a MD preconcentration technique that results in pure water being generated in the permeate. Membranes with CNTs incorporated therein tested under a variety of conditions revealed unexpectedly superior results over membranes without CNTs. Incorporating nano carbons into the structure of a membrane such as for example into the pore structure of a hollow fiber porous hydrophobic membrane can enhance enrichment by membrane distillation of trace quantities of pharmaceuticals in an aqueous solution. At the same time pure water is generated from the waste water.

[0013]In accordance with one embodiment solvent reduction for membranes with CNTs was as much as 28% higher than for membranes without CNTs. The CNTs serve as a means to increase vapor transport capacity of methanol across the membrane.

Problems solved by technology

In part, this is due to the fact that membranes facilitate extraction and separation without allowing the mixing of two phases.

However, the hot solution has a high vapor pressure and MD relies on the net flux of water vapor from the warm side of the membrane to the cool side.

Analytical applications of membrane extraction have relied on the selective permeation of the analyte, and this is a challenge because selective membranes for diverse analytes are not always available.

To date, however, CNTs have not been employed in connection with membrane distillation techniques.

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